Dissertation in Draft: Chapter V

Date:: 1953

Reference:: PP/CRI/F/1/5

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Licence: In copyright

Credit: Dissertation in Draft: Chapter V. Source: Wellcome Collection.

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If we turn a right-handed ci -helix upside down, it stays a right-handed helix, but the direction of the polypeptide chain is reversed in space. The sequence C-N-C-C is like an arrow fixed to the helix to show its direction. We thus have four possibilities for an ©¿-helix whose axis is parallel to some given direction in space, since it must have both its sense of twist and its direction given to fix it unambiguously, apart from the possibility of rotating it about its own axis. These four possibilities are illustra-te^diagramatically in Figure .. 3. THE EQUATORIAL REFLEXIONS It is not proposed to discuss these in detail, especially as the calculations were mainly carried out by the Courtauld workers. It suffices to say that in obtaining a Fourier corresponding to the z projection, one particular combination of signs is probably more correct than any other, and one can therefore obtain the correct Fourier with a fair degree of certainty. For completeness a section through this Fourier is given in Figure . Attempts by the Courtauld 1 s workers and independently by myself, to obtain a helical side-chain arrangement consistent with this Fourier have been uniformly unsuccessful, though one can obtain agreement by assuming that part of the side-chain is highly disordered. Nevertheless the fitting of the equatorial intensities to the «¿-helix allows one to put the experimental intensities approxim ately on an absolute scale, and gives an estimate of the lateral temperature factor or degree of disorder. This can be expressed as exp(~9R 2 ), where R = 2 , and this factor is to be used to modify the structure factors (not the intensities) obtained for point atoms.

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